(1) Field of the Invention
The present invention relates to a process for the preparation of hydrogen ion-exchanged mordenite having a reduced aluminum content.
(2) Description of the Related Art
Zeolite is a generic term for crystalline aluminosilicates and has a chemical composition represented by the following formula based on Al.sub.2 O.sub.3 : EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein
M represents a cation having a valency of n, x is a number of at least 2 and y is a number of at least 0.
Mordenite having a five-membered oxygen ring is a zeolite having an x, that is, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio, of in which x, that is, the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio, at least 10, and mordenites with the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of about 10 to about 30 are now synthesized. Mordenite is characterized by high heat resistance and high acid resistance and is widely used as an adsorbent or a catalyst for the purification of petroleum, the petroleum chemistry and the prevention of environmental pollution. When mordenite is used as a catalyst, it is ordinarily converted to a hydrogen ion-exchanged type and it is used after adjusting the acid intensity by removing a part of the aluminum of the mordenite. This operation of removing a part of the aluminum is called dealumination. The present invention relates to a process for the preparation of dealuminated mordenite that can be used as a catalyst or a catalyst carrier.
As the known processes for the dealumination of mordenite, there can be mentioned a process in which hydrogen mordenite is calcined at a temperature higher than 350.degree. C. and is then acid-treated with 6N HCl under boiling to increase the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to at least 55 (Japanese Examined Patent Publication No. 46-37,166), a process in which mordenite is pulverized and treated with an acid at a temperature of 60.degree. to 100.degree. C. for 10 hours to 14 days to adjust the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to 10 to 45 (Japanese Examined Patent Publication No. 51-30,031), a process in which a steam treatment and an acid reflux treatment are repeated to increase the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to at least 35 (Japanese Examined Patent Publication No. 51-15,000), a process in which a synthesized sodium mordenite powder having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 12 to 30 is treated with an acid having a concentration higher than lN at a temperature of 75.degree. to 125.degree. C. to increase the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to at least 40 (Japanese Unexamined Patent Publication No. 46-3,714), and a process in which hydrogen mordenite is heat-treated at a temperature of at least 600.degree. C. in the presence of steam and is then placed in contact with an acid (Japanese Unexamined Patent Publication No. 58-161,916).
These known processes for the preparation of mordenite have a common problem in that an acid treatment must be carried out under severe conditions for a long time or a heat treatment such as calcination must be carried out in combination with such an acid treatment so as to attain an intended high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio. Accordingly, these known processes are not preferred as an industrial process.